Automatically Deriving Logical, Arithmetic and Timing Dependencies

1. A computer-implemented method for verifying the behavior of a digital system, said method comprising, receiving a constraint graph with directed arcs, each said directed arc being labeled with a Boolean formula, said constraint graph having at least one accepted path, each accepted path in said constraint graph being labeled with a sequence of Boolean formulas describing a disallowed pattern of behavior of said digital system; receiving a list of non-visible signals; starting with said constraint graph, performing for each non-visible signal in the list of non-visible signals: (a) performing either (a1) all possible resolutions of equal-length directed paths within the constraint graph of said non-visible signal or (a2) selected resolutions of equal-length directed paths within the constraint graph of said non-visible signal; (b) deleting all arcs in a resulting constraint graph that are labeled with a Boolean formula that mentions said non-visible signal; (c) deleting all arcs in the resulting constraint graph that are no longer on a directed path from an original initial node of the constraint graph to an original terminal node of the constraint graph; supplying the resulting constraint graph to a constraint-based simulator; supplying input values to said constraint-based simulator; using said input values and constraints accepted by said constraint graph to calculate output values; responding to a user query about a particular output value by determining the input values and the constraint, or constraints, used to calculate said output value; and supplying to the user said input values and said constraint, or constraints.

2. A computer-implemented method for verifying the behavior of a digital system, comprising: a constraint-based inference engine receiving logical/temporal/data dependencies describing the behavior of a digital system; deriving new logical/temporal/data dependencies describing the input/output behavior of the digital system; and wherein said logical/temporal/data dependencies received by and derived by said inference engine are in the form of a directed graph in which each arc is labeled with a Boolean formula.

3. The method of claim 2 wherein said method labels each node of said received directed graph, or a copy thereof, with a data structure.

4. The method of claim 3 wherein a new directed graph with labeled nodes and arcs is obtained from an existing directed graph with labeled nodes and arcs by resolution comprising the following steps: (a) deriving from equal-length directed paths in said existing directed graph a new directed path that is the same length as said equal-length paths such that the Boolean formula labeling the i'th arc of this new path is obtained from the Boolean formulas labeling the i'th arcs of said equal-length paths and the data structure labeling the i'th node of this new path is obtained from the data structures labeling the i'th nodes of said equal-length paths; (b) adding the nodes and arcs of the derived path to the existing directed graph.

5. The computer-implemented method of claim 4 wherein said resolution is via a selected signal S in a selected position k of said equal-length paths.

6. The computer-implemented method of claim 5 , comprising the following steps for a system signal S: (6a) performing selected resolutions via said signal S; (6b) deleting all arcs labeled with a Boolean formula in which signal S appears; (6c) deleting all nodes and arcs not lying on a directed path from an initial node of the original directed graph prior to Steps (6a) and (6b) to a terminal node of the original directed graph prior to Steps (6a) and (6b).

7. The computer-implemented method of claim 6 in which the steps (6a), (6b), and (6c) are repeated for each non-input, non-output signal S.

8. The computer-implemented method of claim 4 in which said method labels each said node with a set of sets of node names of the original received directed graph.

9. The computer-implemented method of claim 4 wherein said method labels each said node with an ordered pairs <aft, fore> where aft and fore are each a set of sets of node names of the original received directed graph.

10. The computer-implemented method of claim 9 wherein said method labels the nodes of said received constraint graph, or a copy thereof, as follows: (a) each initial node is labeled with the ordered pair <{ }, {{ }}>; (b) each internal node N is labeled with the ordered pair <{{N}}, {{N}}>; (c) each terminal node is labeled with the ordered pair <{{ }}, { }>.

11. A computer-implemented method comprising: a constraint-based simulator receiving (a) logical/temporal/data dependencies describing the input/output behavior of a digital system, (b) values for various input signals at various simulation times; and said constraint-based simulator determining values for various output signals at various simulation times using said logical/temporal/data dependencies.

12. The computer-implemented method of claim 11 wherein said received logical/temporal/data dependencies are in the form of a constraint graph.

13. The computer-implemented method of claim 12 wherein said values for various input signals and said values for various output signals are displayed in a two-dimensional tabular form wherein signals are associated with one dimension and simulation times are associated with the other dimension.

14. A computer-implemented method for simulating the behavior of a digital system comprising: receiving a constraint graph with directed arcs in which each said directed arc is labeled with a Boolean formula, said constraint graph having at least one accepted path, each accepted path in said constraint graph being labeled with a sequence of Boolean formulas describing a disallowed pattern of behavior of a digita 1 system; receiving values for selected input signals; using said input values and constraints accepted by said constraint graph to calculate values for selected output signals at various simulation times.

15. The computer-implemented method of claim 14 , further comprising: responding to a user query about a particular output value by determining the input values and the constraint, or constraints, used to calculate said output value; supplying to the user said input values and said constraint, or constraints, in at least one of several alternate formats.